Plunger lift systems are artificial lift systems for oil and gas wells. Plunger lift systems are used during the producing life of the well when the bottom hole pressure and the gas to liquid ratio will no longer support natural flow. A plunger lift system includes a tubing string in the well casing. The tubing string has a well valve and lubricator at the top and a spring assembly at the bottom, and an elongated cylindrical plunger that travels between the bottom and the top of the tubing string. The well is intermitted by shutting in the well for a selected time period to allow pressure build up and then opening the well valve for a selected period of time, allowing fluid to flow into the sales line. The plunger moves up the tubing string during the time the well valve is open, pushing a liquid slug to the top. When the well valve is closed the plunger falls back to the bottom of the tubing string. The open and closed times for the well valve are typically controlled by a programmable controller.
The gas lift plunger in a plunger lift system is the interface between the liquid slug above the plunger and the pressurized gases that push the plunger from below. Plungers are designed to minimize the downward flow of liquid and the upward flow of gas between the plunger and tubing string as the plunger travels up the tubing string. One type of plunger uses mechanical seals spaced along the cylindrical plunger body to seal between the plunger and the tubing string. U.S. Pat. No. 5,253,713 to Gregg et al. discloses a plunger with elastomeric seals. U.S. Pat. No. 5,427,504 discloses a plunger with sealing pads that are biased outwardly against the tubing string by springs. Plungers with mechanical seals provide efficient sealing between the plunger and the tubing string. However, plungers with mechanical seals have a plurality of parts, often making these plungers complex and expensive. These plungers also often require an internal bypass valve that must be opened for the plunger to fall back to the bottom of the tubing string.
Plungers for plunger lift systems have heretofore used the upward gas flow to seal between the plunger and tubing string. This type of plunger has a diameter slightly smaller than the diameter of the tubing string in order to allow liquid to flow upward between the plunger and tubing string while the well valve is closed. These plungers generally have a plurality of longitudinally spaced circumferential grooves spaced between cylindrical body sections of the peripheral surface of the plunger body. The grooves create a turbulent flow in gases flowing upwardly between the plunger and tubing string as the plunger is pushed up the tubing string, and thereby reduce the gas flow up and liquid flow down around the plunger. Prior known plungers have uniformly sized and uniformly shaped grooves uniformly spaced along the plunger body. The recessed surface of the grooves in the prior known plungers connects to the surface of the plunger body along sharp edges or corners. U.S. Pat. No. 4,410,300 to Yerian discloses a plunger with grooves having an asymmetrical V-shaped cross section. U.S. Pat. No. 4,502,843 to Martin discloses a plunger with grooves having a semi-circular cross section. Plungers having grooves with a square or rectangular cross section have also been produced. Plungers with gas flow seals are less complex and less expensive than plungers with mechanical seals. However, prior known gas flow-seal plungers are less efficient in sealing than plungers with mechanical seals.